The template container_value
is a template meta function used as an attribute customization point.
It is invoked by the Qi repetitive parsers (Kleene, Plus,
List, and Repeat) to determine
the type to store in a container.

The customization point is_container
needs to be implemented for a specific type whenever this type is to
be used as an attribute in place of a STL container. It is applicable
for parsers (Spirit.Qi) only. As a rule of thumb:
it has to be implemented whenever a certain type is to be passed as an
attribute to a parser normally exposing a STL container and if the type
does not expose the interface of a STL container (i.e. no embedded typedef
for value_type). These
components have an attribute propagation rule in the form:

This template is instantiated by the library at the appropriate places
while using the supplied container type as the template argument. The
embedded type is used
as the attribute type while parsing the elements to be store in that
container.

The following example shows the predefined specialization for unused_type: